1. Field of the Invention
The present invention relates to an apparatus and process for the delignification of cellulose pulp.
More particularly, the present invention relates to an apparatus and process for delignifying a cellulose pulp slurry at a medium pulp consistency with an alkali and oxygen.
2. Description of the Related Arts
For removing a lignin substance from a cellulose pulp by using an alkali and oxygen, a practical process in which an alkali, oxygen and heating steam are mixed into a cellulose pulp slurry having a pulp consistency adjusted to a medium level of 8 to 15% by weight (based on the dry weight of the pulp), the temperature of the mixed slurry is controlled to 70.degree. to 140.degree. C., the heated mixed slurry is fed under pressure to a delignification apparatus (column), and a desired delignification treatment is applied to the cellulose pulp while the slurry rises from a bottom inlet to a top outlet of the delignification apparatus, is carried out. In this practice, at least two treatment apparatuses (columns) of the above-mentioned type are connected in series and the delignification treatment is applied at least twice to the cellulose pulp slurry.
In the above-mentioned conventional delignification apparatus and process, the rising flow of the mixed pulse slurry fed into the delignification apparatus is likely to differ at the central portion of the treatment apparatus and at portions close to the wall surface of the apparatus. Namely, a high-speed flow, called "channelling", is readily generated in the central portion of the treatment apparatus, and therefore, the portion of mixed pulp slurry flowing through the central portion of the treatment apparatus cannot reside in the treatment apparatus for a desired reaction time and is discharged under an insufficiently reacted condition from the treatment apparatus, while the portions of the mixed pulp slurry flowing close to the wall surface reside in the treatment apparatus for longer than the desired residence time. These uneven flow and residence times of the pulp slurry result in an uneven quality of the resultant delignified cellulose pulp.
To prevent this uneven flow of the mixed pulp slurry, it is widely attempted to arrange a distributor for feeding a mixed pulp slurry and regulating the flow of the fed slurry in the vicinity of the mixed pulp slurry feed inlet of the treatment apparatus, and locating a discharger for discharging (scraping out) the mixed pulp slurry in the vicinity of the mixed pulp slurry discharge outlet.
The arrangement and use of the above-mentioned distributor and discharger, however, result in increased equipment costs and the costs for operating these devices. Moreover, the pressure loss of the mixed pulp slurry in the apparatus becomes large, and thus the maintenance cost of the apparatus is increased.